1. Field of the Invention
The present invention relates to an apparatus for and a method of measuring the birefringence of a detection lens, such as plastics lens, etc. for use in the writing-in or picking-up of a light utilized in a laser printer, etc.
2. Discussion of the Background Art
Up to now, as to a method of measuring the birefringence of a detection object such as a detection lens, etc., a phase modulating method and a rotative analyzer method have been well known. In those methods, parallel light beams are radiated onto a transparent detection object. A transmission light from the detection object is received by a light-receiving element such as a photodiode, etc. The variation of the polarization state of the transmission light due to the birefringence of the detection object is detected, and thereby the birefringence of the detection object can be obtained.
Regarding the phase modulation method, as reported in one background art document "Measurement of Birefringence utilizing the Phase Modulation Method, and Application thereof" on pages 127-134, in OPTICAL TECHNOLOGY CONTACT, Vol. 27, No. 3 (1989), the phase of the radiation light is modulated by use of an optical-elasticity modulator (PEM). The birefringence thereof is obtained from the phase between the beat signal of the light transmitted through the transparent detection object and the modulation signal.
Regarding the rotative analyzer method, as reported in "Polarization Analysis" on pages 256-265, in OPTICAL MEASUREMENT HANDBOOK--Asakura Bookstore (published on Jul. 25, 1981) edited by Toshiharu Takoh, Junpei Tsujiuchi, and Shingeo Minami, etc., an analyzer which is put on a rear surface of the transparent detection object is rotated, and the transmission light is received at the same time by a light-receiving element on the rear surface of the analyzer. The birefringence thereof can be obtained by the variation of the received light output from the light-receiving element due to the rotation of the analyzer.
Furthermore, according to still another background-art document, published specifications of Japanese Laid-open Patent Publication Nos. 4-58138/1992 and 7-77490/1995, enlarged parallel light is radiated onto the transparent detection object and the transmission light transmitted therethrough is received by a two-dimensional sensor. In such a way, the birefringence of the detection object can be obtained, and thereby the surface (two-dimensional) measurement of the birefringence can be realized.
In any one of the phase modulation method and the rotative analyzer method, a so-called "point measurement" is utilized. Namely, fine parallel light beams are radiated onto the detection object and the light beams thus radiated are received by a photodiode, for example. Therefore, in order to measure the entire surface of the detection object, it is necessary to adjust the detection object and the measurement apparatus for measuring the detection object. Especially, in a case that the detection object is a non-flat plate such as a lens, since the light beam radiated onto the detection lens is refracted by the detection lens, the setting operation for the detection object or the measurement apparatus turns out to be very difficult.
Furthermore, according to the published specification of Japanese Laid-open Patent Publication No. 4-58138/1992, the adjustment of the detection object, etc. is not necessary, because of the "two-dimensional" measurement. However, in the case of using a lens having a large diameter such as a writing-in lens (usually an f.theta. lens) for use in a laser printer, etc., the difference between the refractive indexes at the center portion and a circumferential edge portion of the lens is large, and thereby optical distortion tends to occur very often after transmitting the light.
For instance, FIG. 18 shows a structure of a measurement optical system in which the object lens 301 is disposed with a detection lens 300 to construct a focal system. In FIG. 18, collimation light beams 302 are radiated onto the detection lens 300. The light transmitted through the detection lens 300 is collimated by the object lens 301, and thereafter the light thus collimated is guided to the light-receiving element side through a polarization element as the measurement light 303, and then the light thus guided is received by a light-receiving element. The measurement is performed on the basis of the light-receiving output.
On this occasion, the refraction force or the degree of refraction of the light rays 302c passing through the center portion of the detection lens 300 differs from that of the light rays 302e passing through the circumferential edge portion thereof. As a result, in the case of arranging both of the lenses 300 and 301 so as to cause the focuses thereof to coincide with each other, and even if the aberration of the object lens 301 is very small as an ideal lens, the light rays 302e transmitted through the circumferential edge portion of the detection lens 300 are directed toward the side of the light-receiving element as the superposing measurement light 303e. Therefore, it is impossible to obtain clear optical elasticity interference stripe images over an entire surface of the detection lens 300.
FIG. 19 shows an example in which, as the optical elasticity interference stripe images 305 obtained on the light-receiving element 304 in the measurement optical system as shown in FIG. 18, an edge portion 305e of the image 305 becomes brighter than at other portions thereof by the influence of the measurement light 303e due to the superposed light rays, or there exists a portion 306 affected by the stray light. In such a situation, it is difficult to measure the extremely bright portion 305e or the other portions 306 affected by the puzzling light.
In a case as shown in FIG. 20 that a light writing-in lens 400 for use in a laser printer, etc. is the detection lens, in the practical use, the light rays transmitted through the light writing-in lens 400 do not become parallel with the optical axis of the optical system on many occasions, for instance. The example shown in FIG. 20 is an exposure scanning system in which the image surface on the photosensitive body 406 surface is exposed with the light and scanned by the laser light emitted from the semiconductor laser unit 401 through a collimator lens 402, a polygon mirror 403, lenses 404 and 405, and the light writing-in lens 400. Consequently, if the birefringence measurement is practiced with the setting of the measurement optical system so as to make the light rays passing through the light writing-in lens 400 (as a detection lens) parallel with the optical axis of the optical system, the transmission path of the light rays transmitted through the light writing-in lens 400 turns out to largely differ from that in the state of its practical use.
Since the amount of the birefringence is largely changed in accordance with the transmission path of the light rays, it is desirable to practice the measurement of birefringence in the state near the practical use of the light writing-in lens 400. Furthermore, if the transmission light of the detection lens does not become parallel with the optical axis of the optical system, the light is directed slantedly toward the polarization element as the incident light. This results in a measurement error because the polarization has, in general, incident angle dependability.